Thermal imaging devices are devices that are operative to detect radiation in the infrared (IR) range of the electromagnetic spectrum (roughly at 8,000-14,000 nanometer, i.e. 8-14 μm) and produce images of that radiation, known as thermograms. Since infrared radiation is emitted by all objects (which are at temperature of above absolute zero), thermography makes it possible to see the environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature; therefore, thermal imaging allows one to see variations in temperature. When viewed through a thermal imaging device such as a camera, warm objects stand out well against cooler backgrounds.
A thermal image is an image where each pixel represents the temperature of the respective surface area at the target. A digital thermal image is typically an array comprising a plurality of digital pixels. Usually, the number assigned to each pixel is proportional to the temperature at the respective target's surface area that corresponds to that pixel.
Non-specialized CCD (charge-coupled device) and CMOS (complementary metal-oxide-semiconductor) sensors have most of their spectral sensitivity within the wavelength range of 400-1000 nanometers.
Specialized thermal imaging devices use focal plane arrays (FPAs) that respond to longer wavelengths (i.e. 3-5 μm (MWIR), and 8-14 μm (LWIR), i.e. mid- and long-wavelength infrared radiation). The most common types are InSb, InGaAs, HgCdTe and QWIP FPA. The newest technologies use low-cost, uncooled microbolometers as FPA sensors. Their resolution is considerably lower than that of visible cameras and the thermal imaging cameras are much more expensive than their visible-spectrum counterparts.
As mentioned above, the current technology for uncooled thermal imaging used today is microbolometers. A microbolometer is a bolometer used as a detector in a thermal camera. Infrared radiation having wavelengths in the range of 8-14 μm is absorbed by the detector material, heating it up, and consequently causes a change in its electrical resistance. The resistance changes are measured and the measurement results are converted into equivalent temperature changes, which in turn are used for creating a thermal image.
FIG. 1 is an example of a prior art device utilizing the microbolometric technology. The microbolometer sensor is a suspended resistor (referred to as micro-bridge in the Fig.), whose resistance varies with temperature. The read-out circuit is placed below the suspended structure. In most cases, an array of microbolometers is deployed in order to simultaneously generate an entire thermal image, wherein each microbolometer unit is used to generate a single pixel in the array. In other cases, a single pixel microbolometer is used in conjunction with a scanning mirror, thereby generating a virtual array.
However, this technology suffers from a number of drawbacks. Bolometers are typically suspended elements that must be packaged in a vacuum packaging, as it is part of the thermal isolation from adjacent bolometers. The process of fabricating and packaging bolometer arrays (also known as Focal Plane Array—FPAs) is a Micro-Electro-Machining-System (MEMS) process. It is not a CMOS or any other common semi-conductor process. Consequently, there are certain disadvantages associated with the manufacturing of such a device.                a. Production costs are high. A typical FPA cost is ten to thousand times more expensive than a CMOS process for the same die size.        b. Yield is relatively low. The bolometer MEMS structures are delicate and fragile. Moreover, as an FPA is an array of bolometer MEMS sensors, it is a pre-requisite that most of all sensors be functional. This is a difficult requirement to meet that translates to low production yields.        c. Sensitivity and response time. Bolometer sensors are based on gaining thermo-dynamic equilibrium with the respective surface area of the target. As the sensors have certain mass and specific heat capacity, it would take them a certain period of time to reach this state of equilibrium. The time period required for them to reach equilibrium, may be translated into video frame rate, which means that a bolometer imager has limited frame rate due to the sensors' mass and specific heat capacity.        d. Aging and drifting. Bolometer sensors, being fragile MEMS structures, change their electrical and mechanical properties over time. A typical FPA would drift significantly over a period of several years, thus causing degradation of its performance.        e. Lack of uniformity. The process of MEMS bolometer fabrication as mentioned above, results in large variance in performance of the device. This translates into a high degree of lack of uniformity in the image being generated. This lack of uniformity typically requires investing vast efforts for calibrating the device, as well as digital signal post-processing to correct the image being generated.        
In view of the above, there is a need to obtain a low cost high sensitivity thermal imaging sensor that does not have the problems of the currently used sensors.